1. Field of the Invention
This invention relates generally to a film cassette drive mechanism in a dental radiographic apparatus for photographing the entire jaws of the human body and more particularly to a type of dental radiographic apparatus in which the film cassette is of a flat form and can be moved linearly in the direction of rotation of a rotary arm.
2. Prior Art
A description will now be given, by way of example, of this type of radiographic apparatus heretofore in use and the film cassette drive mechanism used therein with reference to FIG. 1 in conjunction with the disadvantages inherent in the apparatus and mechanism.
In the figure, the numeral 1 designates a rotary arm which turns on its axis while describing a curved (for example, an elliptic) orbit approximate to the dental arch by a rotating shaft 23. The arm 1 has an X-ray source 16 and a film holder 13 immovably fixed respectively to one end thereof and to the other end thereof with the source and the holder placed in an opposed relation to each other. A platelike radiographic film cassette 14 is held in the film holder 13 and it is well known that a tomogram is made of the dental arch by the cassette 14 being moved linearly so as to be brought into synchronism with the travelling speed of the X-ray source 16 which follows the rotation on the arm 1. The drive mechanism of the cassette 14 shown in the figure includes a pulley 17 connected coaxially with the rotating shaft 23, a film holder 13 formed into a rectangular box shape having a linear distance sufficient to move the cassette 14 linearly continuously in one direction in the range of a certain speed during a period of time from beginning to start of radiographic photographing, pulleys 18 . . . for a change of direction disposed inside the holder 13, a film cassette 14 likewise disposed in the holder 13 so as to freely travel therein, and endless wires 19 wound on the pulley 17 and distributed in front and in rear in the direction of travel of the cassette by the pulleys 18 . . . for change of direction and connected to the front and rear end faces on the top of the cassette 14, and the drive mechanism functions in the manner that the cassette 14 is caused to continuously travel from one side to the other side of the film holder 13 in response to the rotation of the rotating shaft 17 and the X-ray beams is irradiated from the X-ray source 16 through a vertically long X-ray incidence slit 20 formed in the plate placed in an opposed relation to the holder 13. Incidentally, the numeral 21 designates a cover.
Referring now to the disadvantages inherent in the prior art, one of them is that the use of a pluarlity of sprockets 17, 18 . . . and wires 19 as a means for transferring the cassette 14 increases the number of parts required and makes the construction complicated in mechanism. Another of them is that since the cassette 14 is necessarily slidden into contact with the bottom plate 22 of the holder 13 while the cassette is travelling, such contact imparts a quivering movement to the film cassette, which produces adverse effects on photographing. Still another disadvantage is that, as described hereinbefore, the holder 13 must necessarily be increased in size by the fact that the holder 13 must have a linear distance necessary and sufficient to move the cassette into the holder 13 in the replacement of cassette 14.